Consequences of kelp forest structure and dynamics for epiphytes and understory communities.

摘要

A variety of organisms can generate physical structure in ecological systems. This structure provides numerous other species with refuge from predation and sites for recruitment and growth. In addition to the direct provision of habitat, structure-forming organisms may alter environmental conditions and mediate delivery of abiotic and biotic resources, a phenomenon described as ecosystem engineering. However, the large spatial and long temporal scales over which the dynamics of many of these species occur has made it difficult to test predictions about the effects of structure-forming organisms on associated community members.;I investigated the relationship between giant kelp, Macrocystis pyrifera, a large and dynamic structure-forming organism on temperate reefs, and the species that live beneath its canopy and on its surfaces. In particular, my research focused on how the attenuating effect of kelp on light influences the structure and dynamics of the understory community, and how the dampening effect of kelp on currents influences spatial variation in the abundance and demography of a suspension-feeding epiphyte, the colonial and encrusting bryozoan, Membranipora serrilamella.;A large scale manipulation of kelp on artificial reefs in San Diego, California revealed a negative effect of kelp on algal abundance and light, and a positive effect on sessile invertebrate abundance, which appeared indirect and mediated by shade from the kelp canopy. Analyses of eight years of data at 16 locations on natural reefs in Santa Barbara revealed that the relationships observed experimentally in space, led to predictable patterns through time. In particular, interannual variability in the understory community was positively related to interannual variability in giant kelp abundance.;Percent cover of Membranipora was higher along forest edges than interiors, a pattern that was partially explained by variation in larval recruitment. Through field experiments and observations I found that clonal reproduction was also reduced inside forests, likely because kelp-dampened currents resulted in a lower flux of food to interior colonies. A synthesis of field measurements and predictions of ingestion rates, based on the results of in situ feeding assays, suggested that Membranipora filtration capacity was largely driven by ecosystem engineering effects of kelp forest size and density.